The present invention generally relates to rerouting and change-back systems, and more particularly to a rerouting system and a change-back system for an asynchronous transfer mode (ATM) network which is used in a high-speed broadband integrated services digital network (B-ISDN).
The "rerouting" of the communication path via communication nodes of the ATM network is also referred to as "alternative routing", but the term "rerouting" will be used in this specification. In addition, the "change-back" of the communication path via the communication nodes of the ATM network is also referred to as "reverse of rerouting", but the term "change-back" will be used in this specification. The rerouting is made from an original communication path to a temporary or rerouting communication path when a fault occurs in a line of the original communication path, for example. The change-back is made from the rerouting communication path to the original communication path after the fault is restored.
FIG. 1 generally shows an example of an ATM network. ATM communication nodes 51, 52 and 53 of the ATM network are coupled via real lines. Channels A1 and A2 are allocated for a user A.sub.1 who is connected to the node 51 and to a user A.sub.2 who is connected to the node 52. Channels B1 and B2 are allocated for a user B.sub.1 who is connected to the node 51 and to a user B.sub.2 who is connected to the node 52. Similarly, channels C1 and C2 are allocated for a user C.sub.1 who is connected to the node 51 and to a user C.sub.2 who is connected to the node 53.
Information a1, a2, a3 and b1 transmitted from the users A.sub.1 and B.sub.1 are divided into cells in the node 51, where each cell is an information transfer unit of the ATM. Hence, the cells are multiplexed and transmitted on the real line between the nodes 51 and 52, that is, on the original path which is allocated beforehand. The multiplexed cells include channel identifiers A and B at header parts thereof, and the node 52 identifies the transmitting channels A1 and A2 and the transmitting channels B1 and B2 respectively based on the channel identifiers A and B. As a result, the information field parts a1, a2 and a3 are transmitted to the user A.sub.2 and the information field part b1 is transmitted to the user B.sub.2. Information c1 transmitted from the user C.sub.1 is similarly transmitted on the real line between the nodes 51 and 53, that is, on the original path which is allocated beforehand, and is transmitted to the user C.sub.2.
The header part of the cell also includes an identifier related to the rerouting path which is to be used when a fault occurs in the real line between the nodes. For example, when the path between the nodes 51 and 52 is regarded as the original path, it is possible to allocate beforehand a rerouting path between the nodes 51 and 52 via the node 53. In other words, a virtual network formed by virtual paths and virtual channels can be formed in the network which is made up of the real lines.
When the ATM network shown in FIG. 1 is regarded as the virtual network, the channels A1 and A2 becomes virtual channels A1 and A1 within a virtual path 54 or virtual channels A1 and A2 within a virtual path 55. Hence, the users A.sub.1 and A.sub.2 cannot see and do not need to be conscious of the virtual channels.
When a line fault occurs, for example, the rerouting of the path is made. And when the line fault is restored, the change-back of the path is made.
FIG. 2 shows an example of a conventional rerouting processing unit 60. In FIG. 2, a fault monitoring part 61 detects the generation of the fault and the fault restoration in the virtual path which is coupled to the node. The fault monitoring part 61 notifies to a cell transmission/reception processing part 62 the restored original path in which the fault restoration is made.
The cell transmission/reception processing part 62 manages the virtual paths and the virtual channels at the node. The transmission/reception processing part 62 changes the path in which the cell is transmitted from the rerouted path to the original path and thereafter transmits the cells to the destination using the original path when the restoration of the line is notified from the fault monitoring part 61. In other words, the change-back from the rerouted path to the original path is made immediately when the restoration of the original path is detected.
Generally, the number of nodes in the rerouted path is greater than the number of nodes in the original path. For this reason, the cell which is transmitted via the rerouted path immediately before the change back may arrive at the destination node after the cell which is transmitted via the original path after the change-back. In other words, the order of the cells may become reversed at the destination node immediately after the change-back is made, and in this case an error is generated in the transmitted information.
Unlike the conventional packet switching system, the ATM network is designed to increase the transfer speed of information as much as possible, and for this purpose, the protocol process on the network side is simplified and the switching of the cells is made by hardware. Therefore, no measures are conventionally taken in the ATM network even when the error is generated in the transmitted information after the change-back, and the measures against the error are taken on the user side by use of an error control protocol process.
Therefore, there is a demand to realize a change-back system which can positively prevent the order of the cells from being disrupted immediately after the change-back is made to change back the path from the rerouted path to the original path which is used before the rerouting.
On the other hand, FIG. 3 shows an example of a conventional broadband ISDN. Five nodes 1 coupled via links 2 respectively operate in the ATM mode. A network management part 3 is provided in common to each of the nodes 1, and this network management part 3 is coupled to the nodes 1 via communication lines 5. The link 2 which couples the nodes 1-i and 1-j will be denoted by 2-ij, and the communication line 5 which couples the network management part 3 and the node 1-i will be denoted by 5-i, where i=1, . . . , 5, j=1, . . . , 5 and i=j in this example.
It will be assumed for the sake of convenience that a terminal 4-1 which is connected to the node 1-1 is coupled to a terminal 4-3 which is connected to the node 1-3 via the node 1-1, the link 2-12, the node 1-2, the link 2-23 and the node 1-3, that is, via a logic path VP1. When a fault occurs in the link 2-23 which couples the nodes 1-2 and 1-3 during a communication via this logic path VP1, the nodes 1-2 and 1-3 respectively detect via the link 2-23 that signals from the nodes 1-3 and 1-2 stop. Hence, the nodes 1-2 and 1-3 notify the network management part 3 via the respective communication lines 5-2 and 5-3 that the communication is interrupted.
The network management part 3 manages the rerouting paths for the paths among all of the nodes 1 within the broadband ISDN. Hence, when the network management part 3 is notified from the nodes 1-2 and 1-3 that the communication is interrupted, the network management part 3 notifies the rerouting path (logic path after the rerouting) for the original path (logic path before the rerouting) which passes the link 2-23 to all of the nodes 1 which may set a logic path via the link 2-23 before the rerouting. In other words, the network management part 3 instructs the nodes 1 to carry out a rerouting process for the call which is being set.
For example, the network management part 3 transmits to the node 1-1 via the communication line 5-1 a logic path VP2 which is to be used in place of the logic path VP1 after the rerouting together with a rerouting instruction which instructs the node 1-1 to carry out the rerouting process. In this case, the logic path VP2 extends from the node 1-1 to the node 1-3 via the link 2-14, the node 1-4, the link 2-45, the node 1-5 and the link 2-35.
Each node 1 which receives the logic path used before the rerouting, the logic path used after the rerouting and the rerouting instruction carries out a rerouting process to reset all of the calls which are set via the logic path used before the rerouting to the logic path used after the rerouting.
For example, when the node 1-1 receives the logic path VP1 used before the rerouting, the logic path VP2 used after the rerouting and the rerouting instruction, the node 1-1 detects the calls which are set via the logic path VP1 used before the rerouting. In this case, when the node 1-1 detects a call which is set between the terminals 4-1 and 4-3, the node 1-1 releases the logic path VP1 related to this detected call and resets the logic path VP2 which is to be used after the rerouting. Thereafter, the terminals 4-1 and 4-3 continue the communication via the logic path VP2.
As described above, when the communication via an arbitrary link 2 is interrupted in the broadband ISDN, this interruption is notified to the network management part 3. In addition, the network management part 3 sends the rerouting instruction to all of the related nodes 1 so that the calls which are set via the original logic path used before the rerouting are reset to the logic path used after the rerouting by the rerouting process of each of the nodes 1. As a result, there is a problem in that the rerouting process takes a long time to complete.
Therefore, there is a demand to realize a rerouting system which can carry out the rerouting process within a relatively short time.
As described above, it is important to quickly detect a fault in the transmission path and to reserve a rerouting path so that a communication of a high quality can always be made. In addition, when fault is restored, it is necessary to quickly detect the recovered path and change back the path to the recovered path so that the recovered path can be utilized efficiently for the communication. Furthermore, there is a demand to realize in the general communication network or the switching network a network for making an intercomputer communication, including a communication between a host computer and a terminal. In this case, measures must be taken so that even when a fault occurs within the network the fault cannot be recognized by the external computer.
FIG. 4 is a diagram for explaining a method of detecting a fault between two nodes A and B in an example of a conventional synchronous transfer mode (STM) time division multiplexing (TDM) transmission system. A command is transmitted from the node A to the node B, and a corresponding response is transmitted from the node B to the node A. A timer is started after transmitting the command, and a fault is detected in the line between the nodes A and B if the response from the node B is not received within a specific time T1. A rerouting process is carried out when the fault is detected.
However, the following problems occur when carrying out the rerouting process in the conventional STM TDM transmission system. First, when a relay node exists, it is impossible to notify the need for the rerouting to the destination node. For this reason, the rerouting can only be realized between two adjacent nodes. Second, the phenomenon which is generated by the disconnected line such as the generation of consecutive "1"s on the transmission path cannot be distinguished from the original data. As a result, the disconnection of the line must actually be detected by other means such as a command/response used for the monitoring the line. Accordingly, the time it takes from a time when the disconnection of the line occurs to a time when the rerouting process is actually carried out is relatively long.
On the other hand, when the rerouting is considered in the case of the ATM network based on the prior art, the problems described in conjunction with FIG. 5 occur.
In the case of the ATM network, the data are transmitted in units of cells as described before. When the ATM network includes nodes A, B and C as shown in FIG. 5 and a disconnection occurs in the line between the nodes A and B, for example, the nodes A and B can detect the disconnection of the line. However, the node C cannot detect the disconnection of the line between the nodes A and B because each node has the functions of relaying and switching the cells within the ATM network and empty (or dummy) cells are transmitted from the node B to the node C even when the disconnection occurs in the line between the nodes A and B. In other words, the node C cannot determine whether the empty cells are received because there is no data or because the line between the nodes A and B is disconnected.
Furthermore, it is difficult to quickly change back the path within the ATM network from the rerouting path to the original path after the disconnection is restored for reasons similar to those described above.
Therefore, there is a demand to realize a rerouting system which can quickly detect the disconnection and start the rerouting process, and also notify the disconnection to a node on the end of the path. In addition, there is a demand to realize a change-back system which can quickly change back the path from the rerouted path to the original path after the disconnection is restored.